Correction for spherical and chromatic aberrations in electron lens



July 6, 1948. E. G. RAMBERG 2,444,710

CORREC-TON FOR SPHERICAL AND CHROMATIC ABERRATIONS IN ELECTRON LENS Original Filed Sept. 26, 1942 INVENTOR.

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BY M

Patented July 6, 1948 CORRECTION FOR SPHERICAL AND CHRO- MATIC ABERRATION S IN ELECTRON LENS Edward G. Bamberg, NewYork, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application September 2.6, 1942, Serial No. 459,814. Divided and this application September 27, 1945, Serial No. 618,970

(Cl. Z50-49.5)

6 Claims. l

This application is a division of applicants copending U. S. application, Serial No, 459,814, filed September 26, 1942, upon which Patent 2,401,315 was granted June 4, 1946, entitled Correction for spherical and chromatic aberrations in electron lens, and assigned to the same assignee as the instant application.

This invention relates generally to electron lens Systems and particularly to the correction oi spherical and chromatic aberrations of electron lenses.

electron lenses.

spherical aberration.

rected.

electron objective lens.

ration.

A further object of Similar reference numerals thereto. The negative The distance d The electric field formed 1o Another object is to provide an improved means for and method of correcting spherical and chro- Heretofore various methods have been devised matic aberrations of an electron lens wherein for correcting only for chromatic aberration in electrons irradiating an object are subjected to Such systems have included no the action of a decelerating field which reflects means yfor correcting simultaneously for inherent l5 electrons received from the object and applies In an article by Otto them to an electron lens. Scherzer in Zeitschrift fr Physik, vol. 114, No- .the invention is to provide an imPIOVBd mea/11S vemloei- 13J 1939, pages 427 to 434, entitled The for and method of correcting for spherical and theoretically attainable resolving power of the Chlomat aberrations 0f an electron lens Where' electron microscope,` a system is described for in a Source 0f electrons is focused upon an 0b Correcting only for spherical aberration, in which ject, electrons transmitted by the object are subthe aperture defect (spherical aberration) is suplooted to a decelerating eld, amd Said eld reposedly compensated With electron mirrors or flects said transmitted electrons back through soreeh lenses by Changing the sign of the apela the electron lens, whereby the inherent aberrature defect of the projection lens of the micro, tions Of the llens are Substantially neutralized scope foousing system, rphis method disolosed the aberrations of the decelerating field. Anhy schemer has hot been found very preotioet other Objectis to provide a curved electrode for The instant invention contemplates the siforming one boundary of the above mentioned demultaneous correction for both chromatic and Celelatng eld. spherical aberrations of an electron lens by sub- The TIVSHOH Wi11 be described by reference jecting an electron beam to a decelerating elec- 150 the aCCOmpaTlyng drawing, 0f Which Figure 1 tron held within the focus of the lens to be coris d Schematic diagram for the purpose of explaining theoretically the operation of the sys- 'rne simultaneous correction for both types tem; Figure 2- is a Schematic diagram 0f one of aberration has been found to be dependent embodiment 0f the nVentiOn; and Figure 3 iS upon a predetermined relation between the depth d Schematic diagram of a second embodiment of the electron decelerating field and the focal 0f th@ iIIVEINSOII- length of the lens to be corrected. While simuldre applied to Similar elements throughout the taneously complete correction of both spherical drawing. and chromatic aberrations is difficult in practice, lo Referring t0 Figure 1, an eletIOIl beam flOm complete correction of spherical, and partial cora SOllrCe l S fOCllSed by an electron Objective rection of chromatic aberration is feasible by lens 2 0D an Object 3 diSDOSed in the eld therethe method and means herein disclosed for an of. The positive terminal of a source of pOlen- Likewise, the method tial V is connected to the object 3. The negaand means disclosed may be utilized for correcting do' 'five terminal Of the potential SONIC@ V iS C011- onli.7 for spherical, or only for chromatic abernected to a plane electrode 4 disposed in the path of the electron beam and in a plane BB' Y Among the objects of the invention are to prosubstantially nor-mal vide an improved means for and method of corterminal of the potential source V is also conrccting simultaneously for spherical and chronected to the electron source I. matic aberration in an electron lens system. Anbetween the electrode 4 and the object 3 is other object is to provide an improved means somewhat less than 1A; of the focal length of for and method of correcting simultaneously for the objective lens 2. spherical and chromatic aberrations in an elecbetween the object 3 and the decelerating electrode 4 providesv an electron mirror.

tron lens system which includes a decelerating A virtual where is the angle of inclination of the incident electrons to the axis at the object and image.

Similarly, the chromatic aberration of the decelerating field (the circle of confusion for electrons of kinetic energy V-l-Av at the plane AA' of the object 3) is If We suppose that the focal point of the lens 2 falls in the plane FF (that is, the focal length is'somewhat larger than 4d), the spherical aberration of the lens will be (3) ATS=C93 where C' may be any value larger than onequarter. (See R. Rebsch, Ann. d. Physik, 31, 551, 1938.)

Likewise, the chromatic aberration of the electron lens will be whereK isl less than one but very close to unity. Since f is slightly larger than 4d, it is apparent that the aberrations may be made to substantially neutralize each other.

Referring to Figure 2, a source of electrons, comprising a cathode Ii, irradiates an object 3, comprising a substantially opaque material supported by a conducting film I3. The positive terminal of the source of potential V is connected to the object support I3. The negative terminal of the potential source V is connected to a refiecting electrode l disposed substantially normal to the beam of the emitted electrons and in the same plane as the electron emitting cathode il. The distance d is derived in the same manner as described for the theoretical system of Figure 1. Electrons striking the object 3 are reflected therefrom in the decelerating field between the object support I3 and the reflecting electrode 4 and are again reflected by the electron mirror, formed by the decelerating field, away from the cathode il and reecting electrodev 4. The last mentioned reflected beam passes `through the object support i3 and is focused by the electron lens 2 to form a desired image. It is to be understood that the electron lens 2 may |be of either the electromagnetic or electrostatic types well known in the art. When the distance d, between the object and the reflecting electrode fi', is suitably related to the focal length of the electron lens 2, the resultant spherical and chromatic aberrations of the decelerating eld Will substantially neutralize the inherent spherical and chromatic aberrations of the electron lens 2. The system described has particular utility in the observation of substantially electronopaque surfaces. In a system of this type, correction for chromatic aberration is particularly desirable.

Figure 3 comprises a system, similar to the theoretical system described in Figure 1, wherein an electron source comprising a cathode Il is focused by an `electronlens 2 upon an electron Arca-KfA-UYH 4 permeable object 3 in the field of the lens. The positive terminal of a source of potential V is connected to the object 3 through the object support I3. The negative terminal of the potential source V is connected to a reflecting electrode i disposed at a .distance dl from the object and substantially normal to the axis of the electron lens. The negative terminal of the potential source V is likewise connected to the cathode Il. Electrons transmitted vby the object 3 are reiiected by the reflecting electrode 4 by means of the decelerating field Ibetween the object 3 and the reflecting electrode 4. The reflected electrons are again transmitted either by the object 3, or its support i3, and again focused by the electron lens 2. In a system of the type of Figure 3, correction for spherical aberration is primarily important, while correction for chromatic aberration is of secondary importance.

In each of the systems described heretofore, the correction for aberrations may be somewhat improved by curving the reflecting electrode 4, as shown in Figure 2, to provide additional compensation for electrons at higher angles of incidence, and to adapt the aberra-tions of the reflector to those of a given lens. Furthermore, the distance d may be increased, if desired, by increasing its negative potential with respect to the ypotential at the plane AA of the object 3. With this arrangement, the distance between the object plane and the plane of reversal of the electrons becomes somewhat less than 1A; of the focal length of the lens, while the distance d is merely ldependent upon the value of the potential V.

lThus the invention described comprises a method of and means for correcting simultaneously for spherical and chromatic aberrations of an electron lens, by subjecting electrons focused by the lens to the compensating effects of electronic arrays of the required strength and phase derived from an electron decelerating field. Substantially perfect correction of the chromatic and spherical aberrations of the lens may be obtained [by a predetermined relation between the depth of the decelerating field and the focal length and aberration constants of the electron lens.

I claim as my invention:

1. In an electron beam focusing system for imaging an object including an electron lens and a decelerating electric field, the method of correcting simultaneously for spherical and chromatic aberrations comprising adjusting the depth of said field to have a predetermined relation to the focal length of said lens, irradiating said object, subjecting electrons reflected from said object to said field to again reect said electrons and to derive an electronic image having aberrations of equal value and opposite sign to the aberrations of said lens, and focusing said reflected electrons by said lens to form a corrected image of said object.

2. Apparatus for :correcting simultaneously for spherical and chromatic aberrations' of an electron beam by an electron lens including means for supporting an object, means for irradiating said object, means for forming a decelerating eld, an electron lens, means for subjecting electrons reflected from said object to said field to again reflect said electrons and to derive an electronic image having aberrations of equal value and opposite sign to aberrations in said lens, and means for applying said reflected electrons to said lens to form a corrected image of said object.

3. Apparatus for correcting simultaneously for spherical and chromatic aberrations of an electron beam by an electron lens including means for supporting an object, means for irradiating said object, means for forming a decelerating field, an electromagnetic electron lens, means for subjecting electrons derived from said object to said field to reflect said electrons and to derive an electronic image having aberrations of equal value and opposite sign to aberrations in said lens, and means for applying said reflected electrons to said lens to form a corrected image of said object.

4. Apparatus for correcting simultaneously for spherical and chromatic aberrations of an electron beam by an electron lens including means for supporting an object, means for irradiating said object, means including a curved reector and a source of potential connected between said reflector and the plane of said object for forming a decelerating field, an electron lens, means for subjecting electrons derived from said object to said eld to reflect said electrons and to derive an electronic image having aberrations of equal value and opposite sign to aberrations in said lens, and means for applying said reflected electrons to said lens to form a corrected image of said object.

5. Apparatus for correcting substantially for chromatic aberration and simultaneously partially for spherical aberration of an electron beam by an electron lens including means for supporting an object, means for irradiating said object, means for forming a decelerating eld, an electron lens, means for subjecting electrons reflected from said object to said field to again reflect said electrons and to derive an electronic image having aberrations of equal Value and opposite sign to aberrations in said lens, and means for applying said reflected electrons to said lens to form a corrected image of said object.

6. Apparatus for correcting simultaneously spherical and chromatic aberrations of an electron beam in an electron lens system including means providing an electron decelerating eld, an electron lens, means for successively applying said beam to said field and to said lens, and means for adjusting the depth of said eld to have a predetermined relation to the focal length of said lens to correct simultaneously for the inherent spherical and chromatic aberrations in said lens.

EDWARD G. BAMBERG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,058,914 Rudenberg Oct. 27, 1936 2,126,286 Schlesinger Aug. 9, 1938 2,260,041 Mahl et al. Oct. 21, 1941 2,264,709 Nicoll Dec. 2, 1941 2,372,465 Whittaker Mal'. 25, 1945 2,401,315 Bamberg June 4, 1946 

